Optical disks are widely used in both computer and consumer electronics fields. Compact Disks (CD) were originally produced for storage of audio recordings and later adapted for use on personal computers (PCs) as CD-ROMs. Optical disks are inexpensive to produce and durable. More recently, a next-generation standard for optical disks known as digital-versatile disk (DVD) has emerged. DVD has mush higher data density and bandwidth than CD. Optical disks use a laser beam that is deflected by small pits arranged in a continuous spiral on the disk's surface. The presence or absence of the pits as the disk spins over the laser beam is detected as binary data.
Increasing data bandwidth requirements of PCs has driven development of the higher-speed DVD-ROM readers or drives. Limitations of the technology such as a fixed wavelength of the laser and wobble of the disk as it spins cause data errors at the higher speeds. While the physical data on the disk may be correct, the higher rotational and reading speeds may introduce errors such as jitter. These higher-speed DVD-ROM drives may need to slow down and re-read data sectors when errors are encountered. Re-reading the data defeats the benefits of higher-speed drives.
Markers known as sync marks are added for data synchronization. These sync marks can be detected and used to align the data at the beginning of each physical sector on the disk. Jitter can thus be compensated for, and read integrity improved.
FIG. 1 shows a physical sector with sync marks being read from a DVD optical disk. As is apparent from FIG. 1, DVD sectors are more complex that sectors for CD-ROM. Data stored on a DVD disk is arranged into physical sectors that include sync marks 10. A physical sector has 13 rows, each with two channels of 1456 bits that are each preceded by a sync mark. The 1456 bits are read as ninety-one 16-bit modulated codewords that are later converted by 8/16 demodulation to 91 data bytes (symbols). Each row has two sets of 1456 bits of channel data 14 and two 32-bit sync marks 10. Data read from DVD disk 18 are arranged into physical sectors for error correction.
The sync marks within a physical sector are not identical. Rather, the sync marks follow a definite arrangement within each physical sector. The physical sector begins with a SY0 sync mark. After the first 1456-bits of channel data, the second sync mark SY5 is found midway through the first row. Then after the next channel data, the second row begins with a SY1 sync mark, with another SY5 sync mark midway through the second row. The third row contains SY2 and SY5 sync marks, while the fourth row contains SY3 and SY5 sync marks. The first five rows begin with SY0, SY1, SY2, SY3, and SY4 sync marks. The next four rows begin with SY1, SY2, SY3, and SY4 sync marks, and sync marks SY1, SY2, SY3, and SY4 are repeated a third time for the last four rows. The mid-row sync marks are SY5 for the first five rows, SY6 for the next four rows, and SY7 for the last four rows. Any row in the physical sector can be identified by its two sync marks, since no two rows have the same pair of sync marks. The zero sync mark SY0 occurs only once, at the beginning of the first row of the physical sector. All physical sectors on the DVD disk have this sequence of sync marks.
The DVD data was modulated to minimized voltage excursions that can reduce precision of small-signal circuits in the read channel circuitry. Before data bytes are written to the DVD disk, a modulator expands each byte to a 16-bit codeword by minimizing a running digital-sum value (DSV) that is increased for each one bit and decreased for each zero. The codewords are (2,10) run-length-limited (RLL) codes that have a minimum run of two zeros and a maximum run of ten zeros.
FIG. 2 shows a DVD sync mark. Sync mark 10 is a 32-bit string of bits that are divided into two groups 22, 24. Group 1 (24) is a special sequence of bits that does not occur in the channel data bits. It is a violation of the (2,10) RLL code, since it contains a run of 13 zeros. The sequence of group 1 is identical for each sync mark.
Group 2 (22) occurs first in sync mark 10. Group 2 varies with the eight different sync marks SY0 to SY7. The B bits can be one or zero, depending on the 0:7 sync code. These B bits are also modified for modulation to alter the number of one bits as needed by the modulator. For example, for sync code 0, any of the following group 2 codes can be written to the disk as the SY0 code:
0001 0010 0100 0 PA1 0001 0010 0000 0 PA1 1001 0010 0000 0 PA1 1001 0010 0100 0
These codes contain from two to four one bits, allowing the modulator to adjust the DSV by its choice of sync code. All are detected as SY0.
At higher speeds, soft errors may occur as the data is being read. Error correction is provided for the channel data, but errors in the sync marks could cause the desired data to be missed, requiring that a re-read occur, delaying the data. Sync marks must be quickly detected as the data arrives from the spinning disk, so that buffering can begin at the correct location. For example, the SY0 sync mark at the beginning of the physical sector could be searched for, but if an error occurred while reading the SY0 mark, the sector could be missed.
When a sync mark is missed, a pseudo-sync mark can be inserted into the data stream by a bit counter. Timing windows have been used to re-synchronize data streams when jitter occurs. See U.S. Pat. No. 5,351,231 by King et al., assigned to Cirrus Logic, Inc., and U.S. Pat. No. 5,353,175 by Chiba, assigned to Sony Corp.
What is desired is a sync detector for DVD. It is desired to detect the start of a physical sector when errors are present in the sync marks. An error-tolerant sync detector is desirable. It is further desired to adjust the sync timing for jitter that occurs as the data as is read from the disk. A sync jitter-adjuster that interfaces smoothly with the demodulator is desirable.